The invention relates to a method for representing elastic parameters of object surfaces by scanning the object surface with a focused acoustic beam and detecting the interference between the ultrasonic rays specularly reflected at the object surface and the ultrasonic rays emitted from the object surface after excitation of surface waves.
Scanning of an object surface by means of a focused acoustic beam can be carried out by means of an acoustic microscope operating in a reflection mode. In the normal operating mode, an acoustic lens arrangement is focused onto a certain object plane which is then scanned in a raster-patterned movement perpendicularly to the direction of propagation of the acoustic beam (x/y scanning). The signal reflected by the object is isolated from the emitted signal and used for image representation.
Another known operating mode involves scanning the object at a certain place only in the direction of propagation of the acoustic beam (z scanning). In this arrangement, therefore, the ultrasonic focus is continuously shifted into the depth of the object. The so-called V(z) curves obtained by this means are produced by interference of a specularly reflected ultrasonic beam with an ultrasonic beam which is returning to the acoustic lens arrangement, after being laterally offset, as a result of the excitation of surface waves. The respective output voltage at the acoustic transducer is a reflection signal which corresponds to the integration of the acoustic interference field over the area of the transducer. It is known that materials having different elastic parameters display a V(z) dependence which is characteristic of the material. This is why the curves can be used for identifying homogeneous layers of material.
One advantage of acoustical reflection microscopy consists in the fact that layers lying below the object surface can also be imaged if the acoustic beam is focused onto such a plane. If the beam is focused on the object surface, it is assumed that z=0 for the surface. For focus positions below the object surface, z for the surface is designated to be negative. With different negative z values, the contrasts of the acoustic images is determined by the respective local V(z) value because of the abovementioned interference phenomena. This means that the contrast of the acoustic image must contain information on elastic parameters inside the object surface. A detailed discussion of the relationships between V(z) and the acoustic image contrast is contained in J. Appl. Phys. 49 (10), pp. 5130-5139 (1978).